1. Field of the Invention
The present invention relates to an UpLink (UL) control method and apparatus in a wireless communication system. More particularly, the present invention relates to a UL control method for access of a Mobile Station (MS) to a Base Station (BS) in a wireless communication system.
2. Description of the Related Art
The amount of average data used by mobile communication subscribers is geometrically increasing, and the demand of users for a higher data transmission rate is increasing at a similar rate. To address this growing demand for a high data transmission rate, research is in progress for introducing a communication technology of an ultra-high frequency band of 30 Giga Hertz (GHz) or more, i.e., a millimeter wave system.
In the millimeter wave system, a serious propagation path loss occurs and the coverage of a BS is considerably decreased. To address this issue in the millimeter wave system, it is expected that a beamforming technology for concentrating a transmission/reception power into a narrow space to increase the transmission/reception efficiency of an antenna would be introduced.
FIG. 1 illustrates a wireless communication system supporting beamforming according to the related art.
Referring to FIG. 1, a BS 100 and an MS 102 may transmit/receive data using a plurality of transmission/reception beams each having different directivity. In the system supporting the beamforming, the BS 100 and the MS 102 perform beam training of transmitting/receiving a beam training signal through the plurality of transmission/reception beams having the different directivity and selecting an optimal beam. The BS 100 and the MS 102 perform beam sweeping of transmitting/receiving a beam training signal for all transmissible/receivable beam directions, measuring a reception quality of the beam training signal, and selecting a transmission beam and a reception beam that have the optimal channel environment. In the system supporting the beamforming, the beam sweeping operation is carried out in a process in which a BS and an MS establish a communication channel to start communication. For example, even a transmission beam direction used when the MS firstly performs a random access process is transmitted multiple times in multiple directions using the beam sweeping operation.
FIGS. 2A and 2B illustrate a DL beam training process using a DL reference signal as a beam training signal, and a UL beam training process using a UL random access signal in a wireless communication system supporting beamforming according to the related art.
Referring to FIGS. 2A and 3B, transmission/reception beams of the BS 100 and the MS 102 each are four in number. First, as illustrated in FIG. 2A, to establish a DL channel, the BS 100 transmits a beam training signal, i.e., a reference signal, four times using each of four transmission beams (BS_TX1, BS_TX2, BS_TX3, and BS_TX4) (4×4 times transmission). The MS 102 receives the reference signals, which have been transmitted from the four transmission beams (BS_TX1, BS_TX2, BS_TX3, and BS_TX4), four times using each of four reception beams (MS_RX1, MS_RX2, MS_RX3, and MS_RX4) (4×4 times reception). The MS 102 selects a transmission beam of the BS 100 and a reception beam of the MS 102, which correspond to a reference signal of the best reception quality. This beam training process is identically carried out to determine UL transmission beam and reception beam as illustrated in FIG. 2B.
As illustrated in FIG. 2B, to establish a UL channel, the MS 102 transmits a beam training signal, i.e., a random access signal, four times using each of four transmission beams (MS_TX1, MS_TX2, MS_TX3, and MS_TX4) (4×4 times transmission). The BS 100 receives the random access signals, which have been transmitted from the four transmission beams (MS_TX1, MS_TX2, MS_TX3, and MS_TX4), four times using each of four reception beams (BS_RX1, BS_RX2, BS_RX3, and BS_RX4) (4×4 times reception). The BS 100 selects a transmission beam of the MS 102 and a reception beam of the BS 100 according to a reception quality of the random access signal.
As described above, in the beamforming technique according to the related art, for the sake of a UL transmission/reception beam and a DL transmission/reception beam, an MS and a BS each have to perform beam sweeping of transmitting/receiving a signal for possible all transmission/reception beam directions. However, the scheme in which the MS and the BS to each transmit/receive a signal for all transmission/reception beams has a disadvantage that it is inefficient in an energy efficiency aspect or inter-MS interference aspect. Accordingly, in a wireless communication system supporting beamforming, there is a need to provide a more efficient beam training scheme, and there is a need to efficiently design even a process of UL random access that is transmitted using the beam sweeping operation.
Also, in the millimeter wave system, there is a need to efficiently set power of a random access signal, in order for an MS to overcome a path loss. In the system according to the related art, an MS measures a path loss using a difference between a reference signal transmission strength of a BS and a reference signal reception strength of the MS, and determines a transmission power based on the measured path loss. However, there is a disadvantage that, because this technique is based on a system using an omni antenna, it is difficult to be applied to a beamforming system, which causes a different path loss according to a transmission/reception beam direction.